This invention relates to the field of communications, and in particular to a method and system that tracks phase diverse spread-spectrum signals without the use of phase-locked loops or late-early code-phase tracking.
U.S. Pat. No. 6,985,512, “ASYNCRONOUS SPREAD-SPECTRUM COMMUNICATIONS”, issued 10 Jan. 2006 to Scott A. McDermott and Leif Eric Aamot, and incorporated by reference herein, teaches a communications system that includes a plurality of autonomous spread-spectrum transmitters, and a receiver that detects and demodulates the messages from these transmitters without establishing synchronization in phase or frequency with any of the transmitters. Each of the transmitters and the receiver use the same spreading code and the same nominal channel frequency, but each transmitted signal will have a particular code-phase and frequency relative to the receiver's code-phase and frequency. The baseband signal that contains the multiple transmissions is multiplied by each code-phase of the spreading code to identify active code-phases, i.e. code-phases at which a high correlation to the spreading code is found, and a Fast Fourier Transform (FFT) is applied to the signal at each active code phase to identify the magnitude and phase of the signal at the given active code-phase, from which the value of the information bit can be determined.
U.S. Pat. No. 7,227,884, “SPREAD-SPECTRUM RECEIVER WITH PROGRESSIVE FOURIER TRANSFORM”, issued 5 Jun. 2007 to Scott A. McDermott, and incorporated by reference herein, provides an alternative scheme, wherein the baseband signal is partitioned into individual subsets of the channel, and the active code-phases within each subchannel are detected and demodulated. As illustrated in FIG. 1, with transmitters transmitting at code-phases and frequencies that are independent of the receiver's code-phase and frequency, an example baseband signal at the receiver includes messages 101-106 that occur at different code-phases p1-p6 and frequencies f1-f4. The frequency of the transmitters of messages 101 and 103 are both offset from a reference frequency at the receiver by the same amount f3 and will therefore appear in the same subchannel, but are received at different phases p1 and p3 relative to a reference code-phase at the receiver. Conversely, the messages 105 and 106 are both received at the same code-phase p6 relative to the reference code-phase, but the frequencies of the transmitters of these messages are offset by different amounts f4 and f1 relative to the reference frequency at the receiver, and will appear in different subchannels. By segregating and processing the baseband signal across both frequency and code-phase domains, each of these messages 101-106 can be distinguished from each other and subsequently decoded. U.S. Pat. No. 7,227,884 uses a progressive de-spread transform to efficiently identify active code-phases within the composite signal, and a progressive (or recursive) Discrete Fourier Transform (PDFT) to demodulate each de-spread message distinguished by frequency.
In a conventional spread-spectrum communication system, such as commonly used in cell phone systems, an individual receiver-despreader-demodulator system is allocated to each active transmitter. To compensate for frequency variations in the received signal, such as might be caused by Doppler effects, each receiver typically includes a phase-locked loop to synchronize the receiver to its assigned transmitter. To compensate for phase variations in the received signal, such as caused by differences between the spreading-code rates (‘chip’ rates) at the receiver and transmitter, each de-spreader typically includes a late-early feedback loop, wherein the receiver's chip rate is controlled based on a determined correlation at a later and an earlier code-phase. In this way, the signal that is provided to the demodulator is stable.
In U.S. Pat. No. 7,227,884, a common receiver is used to provide the baseband signal, each subchannel is preferably extracted by a common channelizer, and the de-spreading of each code phase preferably occurs sequentially using a progressive accumulation. In this manner, the cost of receiving and de-spreading a plurality of concurrently transmitted messages is substantially reduced compared to the more conventional spread-spectrum receivers. However, to achieve this cost reduction, conventional transmitter-specific frequency and code-phase synchronization cannot be used. Within a given channel, messages from multiple transmitters, each with its own variations with regard to the receiver, are processed by the same channelizer, and conventional phase-locked loops do not react well to discontinuities. In like manner, if the chip-clock in the de-spreader is advanced to track with one transmitter, it is likely to adversely affect the tracking with other transmitters.
As taught in U.S. Pat. No. 7,227,884, techniques are applied to assess adjacent subchannels and adjacent code-phases to determine if any of the messages in a given subchannel, code-phase bin has ‘slipped’ into a different sampling bin. If so, the messages that are decoded at this new subchannel, code-phase pair are routed to the same output queue as the prior messages. Although this process manages to substantially eliminate discontinuities in the output message queues, there is often a lack of coherency as the signal transitions from one bin to the other, reducing the demodulation efficiency and/or increasing the error rate.
It would be advantageous to compensate for the frequency and/or code-phase variation of received messages while still using common components for detecting messages from multiple transmitters. It would also be advantageous to minimize the transitions of messages from one component to another during the duration of the message.
These advantages, and others, are achieved by using common components to detect messages from a variety of transmitters, and using the characteristics of each detected message to dynamically configure the demodulation process for each message. Channelization is selectively performed before, after, or before-and-after de-spreading, depending upon the particular environment and/or the particular characteristics of the received messages. Frequency synchronization to each transmitter is achieved by initializing a tracking loop within the demodulator associated with the transmitter, and code-phase synchronization to each transmitter is achieved by selectively inserting or deleting bits, based on the frequency and code-phase characteristics of the detected message.
Throughout the drawings, the same reference numerals indicate similar or corresponding features or functions. The drawings are included for illustrative purposes and are not intended to limit the scope of the invention.